1. Field of the Invention
The present invention relates generally to a novel and useful high purity boron nitride and production process thereof. More specifically, the invention relates to a hexagonally crystalline boron nitride which is essentially free of water-soluble impurities, particularly water-soluble boron containing impurities. Further particularly, the invention relates to a hexagonally crystalline boron nitride which can be utilized in various fields, such as ceramic industries, cosmetics industries, electric industries, electronic industries, pharmaceutic industries and so forth.
2. Description of the Background Art
Boron nitride is appreciated as a useful material in various industrial fields, because of its good heat conductivity, electric insulation ability, chemical stability, lubrication ability, lubricity, heat resistance and so forth. Hexagonally crystalline carbon nitride generally has a lamellar crystallographically hexagonal structure like that of graphite. In one application, hexagonally crystalline boron nitride powder is used as an additive in synthetic resins, lubricants and so forth or in solid form as jigs, electric insulators, molds and so forth. The demand for high purity hexagonally crystalline boron nitride has increased in recent years.
For example, the Japanese Patent First (unexamined) Publication (Tokkai) No. Showa 54-163398, discloses a synthetic rubber sheet having high heat conductivity and electric insulation ability and production thereof. Boron nitride powder is utilized as an additive for preparing the synthetic rubber. In the disclosure of the above-identified publication, it is suggested that the surface of the particles of the boron nitride power utilized in preparation of the synthetic rubber should be smooth for obtaining better adherence on the synthetic rubber and to provide high heat conductivity and electric insulation ability. In order to provide the latter property, high purity crystalline boron nitride is required.
On the other hand, Japanese Patent Second (examined) Publication (Tokko) No. 58-181708 discloses a process for preparation of a polygonally crystalline boron nitride by high pressure inversion utilizing boron nitride as starting material. A high yield of polygonally crystalline boron nitride cannot be obtained when the boron nitride starting material, contains oxygen. Furthermore, when oxygen is contained in the boron nitride, strength and optical transparency of the obtained polygonally crystalline boron nitride is degraded. In order to avoid these disadvantages, the disclosure of the above-mentioned publication suggests heating the boron nitride at about 2000.degree. C. to 2200.degree. C. to remove impurities including oxygen to obtain high purity boron nitride.
There are three mutually distinct processes for purifying boron nitride in the prior art. A first process has been proposed in the Japanese Patent First Publication (Tokkai) No. Showa 58-60603. In the proposed process, boron nitride is subjected high temperature to vaporize and/or burn away impurities. A second process, proposed in the Japanese Patent First Publication (Tokkai) No. Showa 59-107907 is to wash out impurities with an alkaline solution. In a third process, the high purity boron nitride is obtained by a chemical reaction between boron trichloride and ammonium, according to the following formula EQU BCl.sub.3 +NH.sub.3 .fwdarw.BN+2HCl
In the aforementioned first process, impurities such as nitric oxide and boron oxide can be vaporized and removed. When the amount of the boron nitride to be treated is small, the vaporized impurities can be removed from the atmosphere by thermal convection and/or diffusion. However, when a relatively large amount of the boron nitride is to be treated creating a relatively large amount of vaporized, decomposed impurities, boron oxide tends to be left in the atmosphere and deposits on the surface of crystalline boron nitride or accumulates between the boron nitride crystallites during the cooling step. Therefore, in processing large amounts of boron nitride, it is not practically possible to satisfactorily remove the impurities in the crystalline boron nitride. Furthermore, since the first process requires high temperatures for removing impurities, it would not be practically applicable in manufacture of high purity boron nitride. Furthermore, even when a small amount of boron nitride is treated for removing oxygen, carbon boron oxide and other impurities, a water-soluble boron compound remains on the surface of the boron nitride crystallites.
In the second process, ions in the alkaline solution are coupled with surface ions of the boron nitride and are retained in the resultant compound. These coupled ions cannot be removed by washing with water. However the resulting high alkalinity of the resultant boron nitride degrades its utility as a material for electronic products. Furthermore, the water-soluble boron compound may not be removed by washing by means of alkaline solutions. The presence of water-soluble boron or boron compounds was proven by the inventor by experimentation. Namely, in the aforementioned Japanese Patent First Publication No. 59-107907, the boron nitride was boiled in water at 100 .degree. C. and 200 .mu.g of boron compound was obtained per each 1g of boron nitride.
The third process may provide high purity boron nitride. However, on the other hand, this process does not provide good crystallization of hexagonally crystalline boron nitride. Furthermore, the boron nitride obtained from the third process has relatively poor electric insulation ability and poor heat conductivity and contains a relatively large amount of water-soluble boron compound.
The water-soluble boron containing component in the boron nitride degrades useful properties of the boron nitride and tends to reduce the number of fields in which it can be used. Therefore, in order to enable the boron nitride to be more widely used, it is preferable to minimize the water-soluble boron content component to obtain highly purified boron nitride.